1. Field of the Invention
The present invention relates to a dynamic pressure bearing manufacturing method for forming a dynamic pressure generating groove on the inner surface of the dynamic pressure bearing, a dynamic pressure bearing manufactured thereby and a dynamic pressure bearing manufacturing device.
2. Description of the Related Art
Recently a dynamic pressure bearing has become used for a spindle motor used for hard disk devices and the like.
A herringbone groove is formed on the inner surface of this dynamic pressure bearing. When a shaft is rotated, lubricating oil is compressed along the herringbone groove and a ring of the compressed lubricating oil is formed in the circumference direction. Thus, the shaft floats off the bearing, and accordingly can be rotated at high speed.
Conventionally, in order to form a herringbone groove constituting the dynamic pressure bearing on the inner surface, an electrode shaped in accordance with the herringbone is formed, and a herringbone groove is formed by transferring the electrode by an electrolytic chemical process, electric discharge process or the like.
Alternatively, a herringbone groove is formed on the inner surface by utilizing spring-back used when applying sizing to a sintered material.
When forming a herringbone groove on the inner surface of the bearing by transferring the electrode by an electrolytic process, spark cutter or the like, an object to be processed is not masked, and also the electrode wears. In this case, if attempting to deepen the groove, the groove also becomes wide. Therefore, it is difficult to set up optimal electrolytic conditions and discharge conditions for that purpose, which is one of its problems.
In the method utilizing spring-back used when applying sizing, since it is difficult to form a groove on an iron-family material by spring-back, the material is limited to a copper-family material with a fairly large modulus of elasticity, and only a shallow groove 3˜4 μm deep can be formed. Besides, if a copper-family material is used, the bearing is damaged by its touching with the shaft at the time of its starting and stoppage, which is another problem.
Alternatively, a herringbone groove can be formed by machining (cutting, forming by rolling, etc.).
However, in the case of cutting, it is difficult to process a material hard to cut, such as highly hard stainless steel, etc., and the life of used tools is reduced, which is another problem. In order to improve process accuracy, tools and using conditions must be managed. Furthermore, since cutting causes burrs, the burrs must be removed.
In the case of forming by rolling, it is also difficult to process a material hard to cut, such as highly hard stainless steel, etc. In order to improve process accuracy, tools and using conditions must also be managed. Since in the case of forming by rolling, swell occurs on the crest part of the groove due to plastic deformation, the swell must be corrected.